1. Field of the Invention
The present invention relates to a method for determining the load in a tumble dryer. The tumble dryer is provided with electrodes for determining the electrical conductivity of laundry in the tumble dryer.
During the drying operation, in addition to the moisture and the type of laundry, which is usually dried in a drum, among other factors, the load in the tumble dryer also plays a major part in the drying process. It is therefore advantageous for various reasons to determine the load in the tumble dryer, for example in order to predict the remaining time accurately, in order to control the drying process in an optimum fashion or in order to output a heating capacity that is matched to the heating requirement.
The prior art discloses methods of determining the load in tumble dryers which are based on the evaluation of temperature measurements or temperature gradient measurements, on the current demand of the motor to move the laundry drum or on a measurement of the electrical capacitance of the drum.
Furthermore, a method for determining the load in tumble dryers in which the electrical conductivity of the laundry is used is disclosed in Published, Non-Prosecuted, German Patent Application DE 29 45 696 A1. According to that document, the drum has fitted in it two electrodes, which touch the laundry from time to time as the drum rotates and by which the electrical conductivity of the laundry is determined. The conductivity signal, which varies with the movement of the laundry in the drum and exhibits pulses, is connected to a threshold-value circuit. The conductivity signal indicates that there is a piece of laundry resting on the electrodes, even in the dry state, by outputting a hit signal of a constant level which is subsequently integrated. The more frequently and the longer a piece of laundry is resting on the electrodes, the more frequent and longer are the hit signals and the higher is the value of the integral of these hit signals. The integral of the hit signals therefore constitutes a measure of the load in the tumble dryer, which can be used to control the drying process.
The above-described method for determining the load in tumble dryers has disadvantages. The formation of the threshold value, with subsequent integration of the hit signals, requires a high outlay of components and circuitry, which is uneconomic and increases the risk of failure. Furthermore, the method functions only if the conductivity falls to a very low value which lies still further below that which corresponds to the conductivity of dry laundry, so that even slight disturbances in the conductivity measurement leads to an erroneous determination of the load. Since the determined conductivity must assume very low values, at least from time to time, in order for the method to function, in the case of a high load, in which a piece of laundry is always resting on the electrodes, a precise determination of the load is no longer possible.
It is accordingly an object of the invention to provide a method for determining a load in a tumble dryer which overcomes the above-mentioned disadvantages of the prior art methods of this general type, which exhibits a high insensitivity to interference, even in the case of a high load, resulting in a reliable determination of the load and with a low outlay for components and circuitry.
With the foregoing and other objects in view there is provided, in accordance with the invention, a load determining method for a tumble dryer, which includes: moving laundry in a laundry drum having electrodes; measuring electrical conductivity of the laundry as a result of contact occurring at least from time to time between the laundry and the electrodes of the laundry drum; and determining a range of fluctuation of the measured electrical conductivity, the range of fluctuation serving measure of a load.
According to the invention, the range of fluctuation of the electrical conductivity is determined and used as a measure of the load. In the case of a low load in the tumble dryer, the probability that no piece of laundry is resting on the electrode is high, with the result that a large range of fluctuation of the conductivity is established. The greater the load becomes, the lower the probability that there is no piece of laundry resting on the electrode, and the range of fluctuation of the conductivity is low.
This method enables the determination of the load with a low outlay, or even without additional outlay, in components and circuitry, since it is based on a conductivity measurement which is implemented in any case in most tumble dryers in order to determine the moisture of the laundry. Furthermore, the method permits a precise determination of the load, even in the case of a relatively high load, since each change in the determined conductivity is registered, without the conductivity having to fall below a specific threshold. This also results in a higher insensitivity with respect to interference.
Advantageously, the range of fluctuation is determined as the difference between the maximum and minimum values of the conductivity within a specific time interval. By restricting the determination of the range of fluctuation to a time interval, it is possible to influence the frequency of the ascertained values of the range of fluctuation by varying the time interval, and to reduce the influence of short-term interference by increasing the duration of the time interval.
Furthermore, the conductivity is advantageously measured at periodic intervals and the range of fluctuation of the individual conductivity measurements is determined. By obtaining discrete conductivity values, it is possible for the determination and evaluation of the range of fluctuation to be carried out without problems, using digital components. Furthermore, by selecting the measuring period, the frequency of the ascertained conductivity values, and therefore the outlay in computing needed for the evaluation, may be determined freely depending on the requirements and options.
Advantageously, the maximum and minimum values of the measured conductivities are averaged and the range of fluctuation of the averages in each case is determined. By averaging, the influence of short-term interference is reduced, and the measurement certainty of the determination of the load is increased. In addition, it is also possible to suppress values that appear to be implausible, such as outliers, and not to take them into account during the averaging process.
It is also advantageous to determine the range of fluctuation several times, to average the ranges of fluctuation obtained and to use the average range of fluctuation as a measure of the load. By this method, the influence of short-term interferences can be reduced, and the measurement certainty can be increased. In addition, it is also possible here to detect values that appear to be implausible, such as outliers, and not to take them into account during the averaging process.
Furthermore, it is advantageous to carry out the determination of the range of fluctuation of the conductivity essentially from the start of the drying process and preferably only over a relatively short period of time, since at this point in time pieces of laundry which dry at different speeds still have an approximately equal moisture, and thus conductivity differences on account of different degrees of moisture do not have a disruptive influence on the determination of the load.
It is particularly advantageous to make use of a microcontroller with an associated analog/digital convertor in order to determine the range of fluctuation of the conductivity. This is particularly advantageous when a microcontroller with associated analog/digital convertor is used in any case to control the tumble dryer, since in such a case no additional outlay of components and circuitry is required at all. Irrespective of this, it is possible in any case to achieve a very high accuracy in determining the load, and this with a relatively low outlay on components and circuitry.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for determining the load in a tumble dryer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.